If medigap policies only cover what medicare covers then why do we need them?

You need it to cover the 20-30% of costs that Medicare does NOT pay for. If you get an Advantage plan it is inclusive so you cannot even buy Medigap, you wont need it.

Medicare doesn't fully cover some things. For many things, if you have only medicare and don't have medigap, then medicare covers a percentage but you pay the rest. If you have medigap, then it pays most of what medicare isn't.

they cover what Medicare does not

What part of Medi GAP is confusing...

They cover the gaps in medicare coverage. Medicare has various out of pocket expenses.

They cover part of the part you'd otherwise pay out of pocket. Medicare doesn't pay 100%.

Because Medicare pays 80% of what Medicare allows. Medigap is to help cover that other 20%.